The field of this invention is the digital radio communication. More precisely this invention relates to a method of channel-adaptive power control as claimed, to a CDMA transceiver device and to a digital CDMA radio communication system as claimed in the independent claims.
Power control method serves to adapt the transmission power of a transmitter in a base station or respectively in a mobile station according to information received from its communication counterpart the mobile station respectively the base station over the radio channel.
In CDMA systems all channel resources are allocated to all users simultaneously. On the one hand the xe2x80x9cNear-Farxe2x80x9d-effect must be properly compensated so that all mobile stations communicating with a base station have approximately the same contribution to the total received signal at the base station independently of their distance to the base station. On the other hand the mobile radio channel has a broad dynamic bandwidth. The characteristics depend directly on the behaviour of the mobile user and are typically affected by radio-link Doppler and Rayleigh-fading effects. Appropriate mechanisms have to be used to avoid connection degradation or loss due to a too high attenuation of the signal w.r.t. interference.
Under these conditions a fast and accurate power control is necessary to ensure an appropriate level of transmission quality for the users. At the same time power control maximises communication density levels in the system. For uplink, this is achieved if the Signal-to-Interference-Ratio (SIR) measured by a base station for each connected mobile station is kept as close as possible to a target Signal-to-Interference-Ratio (SIRtarget).
In prior art e.g. xe2x80x9cCDMAxe2x80x94Principles of Spread Spectrum Communicationxe2x80x9d, Andrew J. Viterbi 1998, Addison-Wesley, Page 182-183, uplink power control has been realised with an open loop power control scheme or with a closed loop control scheme.
In the open loop control scheme a mobile station is tracking a pilot signal sent by the base station while receiving and transmitting traffic. The power level of the pilot signal is in relation with the SIRtarget value and is known by the mobile station. The downlink path loss can then be estimated at any time. Assuming that the physical channel is completely symmetrical, the downlink path loss is the same as the uplink path loss and the mobile station can deduce at any time the appropriate transmission power it has to use.
A closed loop power control takes into account that the dynamical part of propagation loss is not symmetric, since Rayleigh fading depends strongly on the carrier frequency, which may differ considerably in the two directions. In that case the transmission power is based on periodical measurements made at the receiving (base or mobile) station. These measurements lead to an estimation of the SIR of the signal received. SIR sampling is done once per 0.625 ms timeslot as specified in UMTS W-CDMA, ETSI XX.03, Version 1.0.0 (December 1998), Section 5.2.1 (Page 6) for uplink, Section 5.3.1 (Page 9) for downlink. The receiving station compares the estimated SIR with SIRtarget and generates according to an algorithm a Transmit Power Control (TPC) command indicating to which extent the counterpart station has to raise or to lower its actual transmission power. This TPC command is sent back to the counterpart station over a signalling channel. The counterpart station modifies its transmission power in the way indicated by the TPC command. The generation of TPC commands is also done once per 0.625 ms timeslot as specified in UMTS W-CDMA.
In prior art an algorithm named xe2x80x9cbang-bangxe2x80x9d algorithm is used to generate TPCs. It is specified in UMTS W-CDMA, ETSI XX.07, Version 1.0.0 (December 1998), Section 4.1.1 (Page 5) for uplink, Section 4.2.1 (Page 6) for downlink. It simply compares for each sampling instance the estimated SIR with the SIRtarget and generates a binary TPC value equal to xe2x80x9cupxe2x80x9d if the sign of the difference SIRtarget-SIRestimated is positive and to xe2x80x9cdownxe2x80x9d if the sign of the difference is negative. The way of modifying the transmission power is quantified: a Transmission Power Step value (TPS) is fixed. Upon reception of the TPC message, the mobile station converts the value xe2x80x9cdownxe2x80x9d into the indication xe2x80x9creduce transmission power by TPS dBxe2x80x9d and the value xe2x80x9cupxe2x80x9d into the indication xe2x80x9cincrease transmission power by TPS dBxe2x80x9d.
Disadvantages of closed loop power control combined with xe2x80x9cbang-bangxe2x80x9d algorithm are especially detected in urban environments where the mobile channel shows such fast attenuation variations that the loop delayxe2x80x94induced by the measurement in the base station, the TPC command generation, its transmission to the mobile station and the power tuning of the transmitterxe2x80x94causes a noticeable probability that the TPC command at its point of arrival at the transmitter in the mobile station is obsolete. E.g., suppose that the base station considers the received power level to be too high, thereupon generating a TPC command xe2x80x9cdownxe2x80x9d in order to let the mobile station reduce its transmission power. When this TPC command arrives at the transmitter in the mobile station, the attenuation over the radio channel between the base station and the mobile station may have increased to such an extent, that the SIR has dropped below the SIRtarget before the TPC comes into effect. In this case, when the TPC becomes effective, it actually causes the SIR to deviate further from its target value rather than reducing the power error.
In order to reduce these effects, which tend to decrease the potential user capacity in CDMA networks considerably, predictive algorithms could be advantageous but not sufficient. Indeed, such predictive algorithms should be well adapted to one type of channel characteristics, but under other type of channel characteristics, their predictions might deviate considerably from the real appearing values.
It is an object of the invention to provide a method of improving power control in making it possible to control the transmission power in a channel adapting form.
These objects are attained by the features of the independent claims respectively by offering an arbitrary large set of different TPC algorithms (TPCAs) for use. Each TPCA should be well adapted to a special kind of channel characteristics. The set of TPCAs is all the better performing when it covers the largest scale possible of channel characteristics. At any time the transmission power is controlled by the algorithm chosen among the set of algorithms that appears to be optimal for the actual channel characteristics. This method takes advantage of the right algorithm at the right time.
Further advantageous features of the invention are defined in the dependent claims.
Especially an algorithm using a first and second threshold is well adapted to a channel affected by Rayleigh fading.